A number of projects are in progress to evaluate and improve the reliability and validity of dietary assessment. A self-administered diet history questionnaire has been developed and piloted, and evaluations of the food list and quantitation methods indicate a high degree of agreement with a reference method. Field validation studies are under way in several different populations and using several different reference methods. Efforts toward the improvement of nutrient estimation in quantified diet questionnaires have included the development of a nutrient dietant and portion sizes using a data-based approach, based on data from 12,000 respondents in the National Health and Nutrition Examination survey. A data management and diet analysis system for use with the self-administered diet history has also been developed. In conjunction with the U.S.-Finland Beta-carotene/Alpha Tocopherol Lung Cancer Prevention Trial, studies have been designed to assess the validity and reliability of the extensive, self-administered instruments used in that trial. An evaluation of the accuracy of retrospective dietary assessment is being carried out, using subjects who provided multiple seven-day diet diaries seven to fifteen years ago. Their ability to provide information about their diet during that time period will be evaluated using both an interview and a self-administered questionnaire. In addition to the above dietary assessment methodology studies, an effort has been undertaken to evaluate the impact of improved carotenoid analysis on estimated risk levels in epidemiologic studies.